This application is related to the following copending patent applications: application Ser. No. 09/104,612 filed Jun. 25, 1998; application Ser. No. 09/104,789 filed Jun. 25, 1998; and application Ser. No. 09/104,648 filed Jun. 25, 1998, all assigned to the same assignee as the present invention and all of which are hereby incorporated herein by reference.
Ultrasonic instruments, including both hollow core and solid core instruments, are used for the safe and effective treatment of many medical conditions. Ultrasonic instruments, and particularly solid core ultrasonic instruments, are advantageous because they may be used to cut and/or coagulate organic tissue using energy in the form of mechanical vibrations transmitted to a surgical end-effector at ultrasonic frequencies. Ultrasonic vibrations, when transmitted to organic tissue at suitable energy levels and using a suitable end-effector, may be used to cut, dissect, or cauterize tissue. Ultrasonic instruments utilizing solid core technology are particularly advantageous because of the amount of ultrasonic energy that may be transmitted from the ultrasonic transducer through the waveguide to the surgical end-effector. Such instruments are particularly suited for use in minimally invasive procedures, such as endoscopic or laparoscopic procedures, wherein the end-effector is passed through a trocar to reach the surgical site.
Ultrasonic vibration is induced in the surgical end-effector by, for example, electrically exciting a transducer which may be constructed of one or more piezoelectric or magnetostrictive elements in the instrument handpiece. Vibrations generated by the transducer section are transmitted to the surgical end-effector via an ultrasonic waveguide extending from the transducer section to the surgical end-effector.
Sandwich type ultrasonic transducers, also called Langevin transducers, are well known and established for the production of high intensity ultrasonic motion. In United Kingdom Patent No. 145,691, issued in 1921, P. Langevin inventor, a sandwich of piezoelectric material positioned between metal plates is described to generate high intensity ultrasound. Sandwich transducers utilizing a bolted stack transducer tuned to a resonant frequency and designed to a half wavelength of the resonant frequency are described in United Kingdom Patent No. 868,784.
High-intensity ultrasonic transducers of the composite or sandwich type typically include front and rear mass members with alternating annular piezoelectric elements and electrodes stacked therebetween. Most such high-intensity transducers are of the pre-stressed type. They employ a compression bolt that extends axially through the stack to place a static bias of about one-half of the compressive force that the piezoelectric transducers can tolerate. When the transducers operate they are designed to always remain in compression, swinging from a minimum compression of nominally zero to a maximum peak of no greater than the maximum compressive strength of the material.
Other embodiments of the prior art utilize a stud that is threadedly engaged with both the first and second resonator to provide compressive forces to the transducer stack. Threaded studs are also known in the prior art for attaching and detaching transmission components to the transducer assembly. See, for example, U.S. Pat. Nos. 5,324,299 and 5,746,756. Such bolts and studs are utilized to maintain acoustic coupling between elements of the sandwich type transducer or any attached acoustic assembly. Coupling is important to maintain tuning of the assembly, allowing the assembly to be driven in resonance.
Sandwich type transducers are relatively high Q devices, and during operation are driven at resonance, and maintained within a relatively narrow frequency range by feedback control methods known in the art. See, for example, U.S. Pat. Nos. 5,630,420 and 5,026,387 which describe systems incorporating and controlling sandwich type transducers.
It is difficult to manufacture sandwich type transducers due to the high Q/narrow resonance range in which these devices operate. It is common to individually tune every transducer at least once during the manufacturing process. Even with the tight tolerances currently available with modern manufacturing processes, tolerance "stack-up" issues present challenges to designers of sandwich type transducers. "Stack-up" issues occur as normal variations due to combining multiple parts, each part having design tolerances, such that variations due to each part sum together to produce a significant variation.
Currently it is known in the art to design the sandwich type transducer longer than desired for a given resonant frequency. During assembly the sandwich type transducer is tested for its resonant frequency, and then the assembly is trimmed shorter to bring it within the desired tuning range. This trimming process often occurs at attachment surfaces, where other acoustic assemblies such as end-effectors are to be attached. It is known that the surface finish quality at attachment surfaces is an important parameter for efficient acoustic assemblies, and the trimming process adds significant manufacturing issues and expense. See, for example, U.S. Pat. No. 5,798,599, which states that transducers require intimate surface contact between adjacent members, and that this intimacy requires surface finishes within 2 Newtonian rings per inch of flatness.
Thus there is a need for a transducer tuning method that does not require trimming at a contact surface. There is also a need for an acoustic assembly method that can account for variations of frequency resonance of individual acoustic assemblies. It would therefore be advantageous to eliminate the need for trimming of acoustic assemblies. It would further be advantageous to be able to design sandwich type transducer components to the desired length for resonance without adding length for tuning due to tolerance "stack-up" issues. It would also be advantageous to provide a method of tuning acoustic assemblies during manufacture that was capable of tuning high Q resonant devices from an existing resonant frequency to a desired resonant frequency. This invention addresses and solves these needs as described below.